Ken Sembach 26-October-2005

1. HST Two-Gyro Mode Ken Sembach 26-October-2005 Two-Gyro Science Mode Website http://www.stsci.edu/hst/hst_overview/TwoGyroMode (includes links to ISRs)

2. David Adler Santiago Arribas Louis Bergeron Carl Biagetti Michael Bielefeld John Biretta John Boia Gary Bower Mike Boyer Stefano Casertano Don Chance George Chapman Kerry Clark Colin Cox Ilana Dashevsky Roelof de Jong Rob Douglas Ron Downes Rodger Doxsey Harry Ferguson Leslie Foor Mary Galloway Mauro Giavalisco Ron Gilliland Mark Giuliano Steve Handy William Hathaway Inge Heyer Bill Januszewski Danny Jones Ian Jordan Anton Koekemoer Vera Kozhurina-Platais John Kucel John Lecourt Andy Lubenow Ray Lucas Jack MacConnell Jennifer Mack Sangeeta Malhotra Jinger Mo Carey Myers Ed Nelan Keith Noll Alan Patterson Marc Postman Cheryl Pavlovsky Chien Peng Karla Peterson Beth Perriello Rick Perrine Lee Quick Merle Reinhart Adam Riess Christine Ritchie Tony Roman Tricia Royle Susan Rose Kailash Sahu Al Schultz Ken Sembach Marco Sirianni Galina Soutchkova Scott Stallcup Denise Taylor James Taylor Kelli Underwood Alison Vick Alan Welty Brad Whitmore Tommy Wiklind William Workman Chun Xu Jim Younger STScI Two-Gyro Mode Team Effort ACS analysis team NICMOS analysis team Schedulers

3. Two-Gyro Mode • HST uses gas bearing rate-sensing gyros to provide information about changes in observatory pointing. • Gyros do not change the pointing. Reaction wheels provide the torques needed to change the pointing. • The HST attitude control system was originally designed to operate with 3 gyros. • 4 of 6 gyros presently installed in HST are functional • To conserve gyro lifetime and extend the life of the HST mission, HST was preemptively placed in two-gyro mode on 8/28/05. • Gyro #4 was turned off 09/01/05 • Gyro #6 was already off • Gyros #1 and #2 are currently on • The FGS provide the missing (orthogonal) axis of control during science observations.

4. Two-Gyro Operations - Key Points • Science data appear nominal and reveal no significant anomalies. • HST instrument performance in two-gyro mode is essentially indistinguishable from performance in three-gyro mode. • Observations requiring the finest pointing control (coronagraphy and high-resolution imaging) are feasible. • Moving targets have been observed (Mars, Uranus). • Fine-pointing jitter is typically ≤5 milli-arcseconds (RMS over 60 sec interval). • Scheduling is more restrictive in two-gyro mode because entry into fine pointing mode for science observations is more complicated. • Only about 50% of sky available at any given time • Not allowed • Gyro-only tracking • Guide star handoffs • Single guide star acquisitions

5. Jitter in Two-Gyro Mode • Pointing jitter derived from inputs into the two-gyro attitude control law is comparable to the jitter in three-gyro mode. • This amount of jitter is hard to detect in the science data. Data provided by B. Clapp (LMCO)

6. Three-Gyro (top) vs. Two-Gyro (bottom) 90 70 50 30 10 -10 Solar Avoidance Zone -30 -50 -70 90 Declination (degrees) Unavailable region 70 50 30 10 Solar Avoidance Zone -10 -30 -50 -70 0 50 100 150 200 250 300 350 -90 Right Ascension (degrees) Two-Gyro Scheduling • Scheduling efficiency in two-gyro mode is slightly lower than in three-gyro mode (~72 vs. ~80 prime orbits/week). • For unconstrained observations, any point in the sky is available at some time during the year. • For constrained observations, placing limits on roll angle or time of observation restricts availability. • Consult the two-gyro website for detailed scheduling graphs and tables.

7. Two-Gyro Science Mode Orbital Verification (TGSMOV) • On-orbit tests during the transition to two-gyro mode verified ACS and NICMOS instrument performance. • ACS: #10458-10461 • NICMOS: #10462,10464 • Tests for • PSF width • Pointing stability • Coronagraphy • Moving target tracking A 10-second ACS/HRC/F555W observation of the globular cluster NGC 2298 taken in two-gyro mode.

8. ACS PSF Analysis • Multiple exposures of three rich star clusters with HRC F555W • Sequences of 10, 100, 500 sec exposures • Slight dependence of PSF width of exposure duration • FWHM measurements for stars with S/N > 10. • Hundreds of stars per image • Bright and faint guide stars to check results • V = 13 and V = 14 • No dependence of PSF width on GS magnitude seen

9. ACS Instrument Performance in Two-Gyro Mode is Excellent 186 HRC exposures Min (FWHM) = 1.89 Avg (FWHM) = 2.00 Max (FWHM) = 2.19 PSF analayses by M. Sirianni, C. Pavlovsky, R. Lucas Three-gyro historical data Avg (FWHM) = 2.04±0.03 August 2005 114 exposures 3 clusters - NGC 2298 - NGC 1891 - NGC 6752 October 2005 72 exposures 1 cluster (CVZ) - NGC 6752

10. Histogram of all exposures NGC 6752 (October) Min = 1.89 Avg = 1.97 Max = 2.06 NGC 6752 (August) Min = 2.04 Avg = 2.09 Max = 2.19 NGC 6752 Two-Gyro Observations August 2005 18 exposures Non-CVZ time Sun Angle ~ 115˚ October 2005 72 exposures CVZ time Sun Angle ~ 80˚ Sun Angle for the two other clusters was ~70˚ and ~89˚.

11. PSF Dependence on Texp Longer exposures have slightly broader PSFs. The longer exposures were taken later in each orbit. 500 sec FWHM = 2.083±0.046 100 sec FWHM = 2.046±0.030 10 sec FWHM = 2.023±0.033

12. PSF Dependence on Time in Orbit HRC TGSMOV PSF & Breathing Model • PSFs get broader with time in individual orbits. • PSF variation is larger than expected simply from exposure time differences. • Dependence is likely due to normal changes of focus caused breathing cycle of the telescope during the orbit. HRC PSF FWHM Measurements - Day 241 Breathing model focus change 2.1 HRC FWHM (pixels) 1.9 MJD (29 August 2005) Analysis by Matt Lallo.

13. ACS - Pointing Stability Test

14. ACS - Pointing Stability Test Pointing stability in two-gyro mode is indistinguishable from stability in three-gyro mode.

15. Two-Gyro Moving Target Test • 32 x 0.3 sec F435W HRC images of Mars over 1 orbit. • Median ~ 10000 e-/pixel • Up to 30000 e- in icecap • Rotation of Mars complicates cross correlation of images to find shifts • Made mask (> 5000 e- =1 < 5000 e-=0) and cross correlated masks with drizzle tools to find shifts.

16. Mars Position Measurements • Compared measured shifts of Mars image to expected shifts from the difference between predicted and final HST ephemeris. • Direction and scale of shifts agree, but small differences of ~16 mas remain. • Residuals are smaller than the unavoidable errors from in-track HST positional uncertainties. Analysis by C. Proffitt

17. ACS Coronagraphy • Coronagraph spot jumps unpredictably by up to 3 HRC pixels between visits. • Variation of spot position is more significant than two-gyro pointing uncertainties. • Earth flats taken weekly to measure position offset. • Offset chosen for subsequent observations to minimize position error. • Coronagraphic test compared coronagraph images through four filters. • Three-gyro mode, September 2002 • Two-gyro mode test, February 2005 • Two-gyro mode, August 2005 • No significant differences found between two-gyro and three-gyro modes.

18. ACS Coronagraphic Images HD 130948A HD 130948A HD 216149 F625W F625W F625W Three-gyro image September 2002 Exposure 30 sec Two-gyro image February 2005 Exposure 300 sec Two-gyro image August 2005 Exposure 300 sec

19. ACS Coronagraphic Image Radial Profiles Analysis by C. Cox

20. NICMOS Coronagraphy F110W F160W Observations of HD 17925, (G star, V=6.0) Direct images Acquisition successful, repeatable Coronagraphic images See NICMOS ISR 2005-001 (Schultz et al.) for analysis of similar observations in February 2005.